Floods and Flash Floods: Hazards in Oklahoma
Nationally, flooding and flash flooding is the number one killer of all storm-related natural
disasters. An average of about 140 people die each year in the United States due to flooding and
flash flooding. In Oklahoma, flash flooding is a consistent threat to life.
This document will focus on flooding caused by excessive precipitation. Flooding as a result of
large structural failures will be treated in a separate document.
Definitions: Flash Flooding versus River Flooding
Flooding occurs on continuous scales of time and area. Humans tend to group events into two
categories: flash flooding and river flooding. Flash flooding refers to events that occur during or
immediately after the life cycle of a thunderstorm. These events respond on the order of minutes
to hours to heavy precipitation. River flooding refers to the response of larger streams to
prolonged precipitation. Flooding on this scale may take days, or even weeks, to culminate.
Vulnerability to flash flooding and river flooding is a dichotomy, in that flash flooding’s primary
threat is to human life and safety, while river flooding’s primary threat is substantial economic
The size of a stream’s watershed is the dominant factor in the time scale of its response to heavy
precipitation. For example, very small creeks and branches of creeks can respond in minutes to
heavy precipitation. Larger rivers, such as the Arkansas, may take days to crest after prolonged
periods of rainfall.
Characteristic or Flash Flooding River Flooding
Size of waterway Small (from unnamed creeks to Large creeks, minor rivers, major rivers
Area of watershed Several hundred acres to tens of About a hundred to several thousand square
or catchment square miles miles
Associated Convective storms, often slow- Larger-scale precipitation patterns, often
precipitation moving or “training” thunderstorms slow-moving fronts or remnants of tropical
storms, or both
Duration of event Minutes to hours Hours to days
Time scale of Minutes to hours Hours to days
Warning Lead Short (no warning to several minutes) Significant (several hours to about a day)
Location of Within or very near the footprint of Often downstream of heavy rainfall
flooding heavy rainfall (increasingly so with larger watersheds)
Primary threat Loss of life; human safety Loss of property; economic disruption
Table A. Typical characteristics of, and factors related to, flash flooding and river flooding in Oklahoma.
Flash flooding occurs when the precipitation rate becomes so large that waterways cannot
evacuate the runoff, streams swell and flash flooding occurs. It can occur as soon as minutes
after a downpour has begun. The conditions that lead to flash flooding can happen anywhere in
Oklahoma, during any season, and at any time of day.
Flash flooding is a real and significant hazard to life in Oklahoma because of two major factors:
1. The prevalence of convective precipitation. Thundershowers and thunderstorms can
produce precipitation at very high rates. These events are certainly a large part of
Oklahoma’s rainfall climate. They are especially prevalent during the warm season.
2. Vehicular travel.
People often underestimate the power of water. This leads to many unfortunate and sometimes
tragic encounters during seemingly minor flooding events. As little as six inches of water can
cause a driver to lose control of a passenger vehicle.
From 1960-2002, there were 94 deaths due to flash flooding in Oklahoma. The vast majority of
these deaths are vehicle-related. Since 1994, all flash flood deaths in Oklahoma have been
Precipitation The most obvious contributing factor. As the rate of precipitation increases, so to does its
Rate ability to outpace the ability of the watershed to absorb it. This is the dominant factor in
flash flooding events, and can overwhelm any or all of the following factors.
Training Storm cells that follow each other (much like box cars on a train) can repeatedly deposit
Echoes large amounts of water on the same watershed, overwhelming its ability to handle runoff.
Slope of Steeper topography (hills, canyons, etc.) will move runoff into waterways more quickly,
Watershed resulting in a quicker, flashier response to precipitation.
Shape of Longer, narrower watersheds will tend to “meter out” runoff so that water arrives from
Watershed downshed (nearer to the mouth of the stream) areas faster than from upshed areas. In
watersheds that are more square or circular than elongated, runoff tends to arrive in the
main stem at the same time, intensifying the response. This factor becomes more
significant with larger watersheds.
Saturation Saturated or near-saturated soils can greatly reduce the rate at which water can soak into
of Soils the ground. This can increase runoff dramatically.
Hardened Extremely dry soils can develop a pavement or “crust” that can be resistant to infiltration.
Soils This is especially true in areas of recent wildfire, where plant oils or resins may cause the
soil to be even more water-resistant.
Urbanization The urban environment usually intensifies the response to heavy precipitation. The two
dominant urban factors are: 1) increased pavement coverage, which prevents infiltration
and dramatically increases runoff; and 2) Urban systems are designed to remove water
from streets and byways as quickly as possible. This accelerates the natural response to
precipitation by placing runoff in waterways much more quickly.
Low-water The vast majority of flash-flood related deaths occur in vehicles. Many of these deaths
crossings occur at low-water crossings where the driver is unaware of the depth of the water or the
consequences of driving into it.
Table B. Contributing factors to flash-flood hazard and vulnerability in Oklahoma.
Figure 1. Flood and flash-flood related deaths in Oklahoma and nearby states, 1961-1999. Single
events strongly influenced totals in Colorado (Big Thompson, 1976, 140 deaths), Louisiana
(Hurricane Betsy, 1965, 50+ deaths), and Mississippi (Hurricane Camille, 1969, ~130 deaths).
Figure 2. Flood-related deaths in Oklahoma by year since 1960. The vast majority of these deaths
are flash flood deaths, and the vast majority of those are vehicle-related.
Flooding of larger streams and rivers typically requires many hours (a day or more) of
intermittent or continuous heavy precipitation over a larger area. Typical scenarios for river
flooding may involve a stalled or slow-moving front with persistent associated precipitation, the
remnants of a land-falling tropical storm, or interaction between these two features.
River flooding occurs when heavy runoff from several tributaries converge in a larger stream’s
main channel. The stream level rises, crests and drops over the course of hours or days. As a
general rule of thumb, the response to precipitation is slower for larger rivers. River flood
damage occurs over a wide area, sometimes several square miles.
Oklahoma’s vulnerability to river flooding changed dramatically during the last half of the 20th
Century. Several factors combined to minimize the loss of life due to river flooding:
1. Physical Floodwater Control – Widespread damming of rivers and upstream tributaries
has dramatically reduced the frequency and magnitude of river flooding in Oklahoma.
2. More Accurate Forecasting – Hydrological forecasting has improved, as has the
timeliness and availability of rainfall observations. As a result, the forecast level of larger
streams is much more predictable. River stage forecasting has matured to levels of
accuracy that were impossible early in the century.
3. Longer Warning Lead-Times – Because river flooding typically occurs hours to days after
rainfall ceases, warnings for river flooding often provide much more lead time than those
for flash flooding.
4. Floodplain Management – The state, through the Oklahoma Water Resources Board, has
aggressively pursued a policy of mitigation through incremental reclamation of flood-
prone areas. This has gradually reduced the number of residences in harm’s way.
As a result of these factors, the primary vulnerability to river flooding is economic in nature.
Most major economic damage is confined to a few very large events. For example, flooding
during the three years of 1957, 1984 and 1986 caused more damage than the remainder of the
years between 1955-1999 (see Figure 3).
Figure 3. Economic damage due to flooding in Oklahoma: 1955-1999. River flooding events in
1957, 1984 and 1986 constitute the majority of dollars during the period. From Climatological
Data National Summary, Annual Summary 27(13), 124; U.S. Army Corps of Engineers (1992)
Seasonal Trends in River Flooding
Spring and fall are the preferred seasons for river flooding, because these are the seasons that
provide the bulk of Oklahoma’s rainfall. However, river flooding can occur during any month on
Autumn months in Oklahoma provide an enhanced threat of wide-scale flooding because of the
combination of several contributing factors. Late summer and autumn are the peak of the Gulf
hurricane/tropical storm season. Large-scale weather patterns (fronts and upper-level storms) are
also much more active in the autumn than in the summer. Moisture from landfalling tropical
storms can interact with slow-moving fronts to provide heavy rains for days at a time. In
Oklahoma, these are the ingredients for river flooding. The moisture can be provided by tropical
storms in the Gulf of Mexico, or even from the remnants of Pacific tropical storms (see Table C).
Year Month Tropical Source Comments
1996 September Fausto Pacific 6+ inches rain; minor flooding along North Canadian.
1995 August Dean Gulf of 12-16 inches in parts of OK; interacted with weak, stalled cold
Mexico front; major flooding along much of Salt Fork of the Arkansas
River in Grant and Kay Counties; flooding also occurred on
Cimarron, Washita and Arkansas Rivers.
1988 September Gilbert Gulf of Interaction with slow-moving front; 4+ inch rains fell onto
Mexico saturated soils; flooding on creeks and rivers.
1986 September- Paine Pacific Up to 20 inches in north-central OK; massive flooding on
October Cimarron. flooding on the Arkansas River; ground was already
saturated by rainfall associated with remnants of Pacific
Hurricane Newton; estimated damages of $350 million; 52
counties declared disaster areas.
1983 October Tico Pacific Up to 17 inches rain in southwest and central OK; Red River
at Burkburnett and Terral rose to highest stage in 60 years;
widespread flooding of smaller rivers and creeks.
1981 October Norma Pacific Up to 24 inches of rain in south-central OK (Monthly total of
25.8” at Madill is greatest for any station during any month in
1961 September Carla Gulf of
Table C. Selected Tropical-Storm-Related River Flooding in Oklahoma.
Appendix A: Oklahoma Flash-Flood Warnings and Events, 1986-2003
The following list contains the total number of flash flood warnings and verified flash flood
events for each of Oklahoma’s 77 counties. Warnings indicate the number of times that
meteorological conditions indicated an imminent threat of flash flooding. Verified events are the
number of flash flood events that were confirmed by National Weather Service (NWS) personnel
in follow-up investigations.
Discrepancies between the two sets of numbers are indicative that not all flash-flood events
receive warnings, and not all warnings have subsequent flooding. Flash flood forecasting is
difficult and limited somewhat by observational technology. Flash flood events in sparsely
populated areas are more difficult to verify than those in urban areas. During times of widespread
severe weather, flash flooding is only one of several hazards that threaten the public, and must
“compete” with violent weather for the time and attention of NWS staff.
Warnings Verified Grady 49 13 Nowata 31 25
County Issued Events Grant 41 26 Okfuskee 20 17
Adair 33 19 Greer 23 8 Oklahoma 65 49
Alfalfa 31 22 Harmon 14 8 Okmulgee 38 32
Atoka 32 15 Harper 7 3 Osage 71 54
Beaver 11 4 Haskell 31 24 Ottawa 23 18
Beckham 18 5 Hughes 12 5 Pawnee 37 21
Blaine 21 10 Jackson 32 15 Payne 20 12
Bryan 57 26 Jefferson 29 13 Pittsburg 52 39
Caddo 40 19 Johnston 25 9 Pontotoc 17 5
Canadian 49 20 Kay 63 46 Pottawatomie 22 13
Carter 32 14 Kingfisher 45 21 Pushmataha 48 18
Cherokee 46 34 Kiowa 37 16 Roger Mills 11 4
Choctaw 38 16 Latimer 45 32 Rogers 44 38
Cimarron 9 10 Le Flore 71 36 Seminole 16 11
Cleveland 37 25 Lincoln 26 18 Sequoyah 34 20
Coal 17 5 Logan 4 21 Stephens 44 31
Comanche 44 30 Love 26 6 Texas 13 8
Cotton 30 12 Major 17 10 Tillman 30 14
Craig 27 20 Marshall 33 13 Tulsa 74 61
Creek 49 42 Mayes 26 15 Wagoner 38 27
Custer 18 10 McClain 30 8 Washington 29 27
Delaware 21 18 McCurtain 71 29 Washita 19 11
Dewey 13 6 McIntosh 28 21 Woods 14 11
Ellis 16 7 Murray 26 16 Woodward 14 8
Garfield 36 21 Muskogee 48 33 Statewide 2495 1470
Garvin 17 9 Noble 26 12